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United States Patent |
5,613,993
|
Richards
|
March 25, 1997
|
Process for encapsulating a shaped body for hot isostatic pressing by
sol-gel method
Abstract
There is disclosed a process for encapsulating a preform of arbitrary size
and shape for the purpose of manufacturing a body by means of hot
isostatic pressing. The preform is encapsulated with glass using a sol gel
technique. The glass initially forms a hard, porous layer. Heating the
glass in the hot isostatic press results in the formation of a
gas-impermeable capsule around the preform.
Inventors:
|
Richards; Kerry (Abingdon, MD)
|
Assignee:
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The United States of America as represented by the Secretary of the Army (Washington, DC)
|
Appl. No.:
|
521394 |
Filed:
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August 29, 1995 |
Current U.S. Class: |
65/17.2; 427/227; 427/228; 427/296; 427/376.2 |
Intern'l Class: |
C03B 008/00 |
Field of Search: |
427/227,228,296,376.2
65/17.2
|
References Cited
U.S. Patent Documents
4340619 | Jul., 1982 | Gaul et al. | 427/228.
|
4476626 | Oct., 1984 | Moritoki et al. | 65/18.
|
4540803 | Oct., 1985 | Cannady | 556/412.
|
4756977 | Jul., 1988 | Haluska et al. | 428/704.
|
5436083 | Jul., 1995 | Halusha et al. | 428/688.
|
Other References
Sol-Gel Processing of Complex Oxide Films, by Guanghua Yi and Michael
Say American Ceramic Society Bulletin, vol. 70, No. 7, pp. 1173-1179 Jul.
1991.
|
Primary Examiner: Czaja; Donald E.
Assistant Examiner: Ruller; Jacqueline A.
Attorney, Agent or Firm: Krosnick; Freda L., Roberto; Muzio B.
Claims
I claim:
1. A process for encapsulating a preform, prepared from a powder, with
glass for the purpose of preparing a shaped body by hot isostatic pressing
comprising the steps of:
preparing a mixture comprising a sol and a filler glass having an identical
composition as the sol, which gels upon application to a preform,
the glass fillers selected from the group consisting of a high-temperature
filler and a low-temperature filler:
coating an outer surface of the preform with at least one layer of the sol
mixture;
after the mixture has gelled, drying the sol gel coating;
baking out the coated preform;
heating the coated preform under vacuum and at temperature sufficient to
cause the sol gel coating to seal the outer surface of the preform; and
subjecting the sealed preform to hot isotstatic pressing at high
temperatures and pressures in an inert atmosphere.
2. The process according to claim 1, wherein the preform is coated with a
plurality of layers of the mixture, and each of the layers is separately
dried after each respective layer gels to the preform.
3. The process according to claim 2, wherein each layer of the coating is
dried at ambient pressure and temperature.
4. The process according to claim 1, wherein the bake-out step is performed
at a temperature ranging from 500.degree. C. to 800.degree. C.
5. The process according to claim 1, wherein the sealing step is performed
at a temperature ranging from 800.degree. C. to 1500.degree. C.
6. A process according to claim 1, wherein the filler is selected from the
group consisting of a high-temperature filler and a low temperature
filler.
7. The process according to claim 6, wherein the high-temperature filler is
selected from the group consisting of, fused silica scrap, and silica
powder.
8. The process according to claim 6, wherein the low-temperature filler is
a glass comprised primarily of silicon dioxide, aluminum oxide, boron
oxide, and sodium oxide.
9. process for encapsulating a preform with glass for the purpose of
preparing a shaped body by hot isostatic pressing, comprising the steps
of:
preparing a first sol which gels on application to a preform comprising a
mixture of a high-temperature sol and a high-temperature glass filler:
preparing a second sol which gels on application to a preform comprising a
mixture of a high-temperature sol and a low-temperature glass filler;
forming an inner layer by coating the preform with at least one layer of
the first sol, and after the first layer has gelled, drying the first gel
coating;
forming an outer layer by coating the preform with at least one layer of
the second sol, and after the second sol layer has gelled, drying the
second gel coating;
baking out the preform;
heating the coated preform under vacuum and at a temperature sufficient to
cause the second sol gel coating to seal the outer surface of the preform;
subjecting the sealed preforms to hot isostatic pressing at high
temperatures and pressures in an inert atmosphere; and
recovering the preforms.
10. A process according to claim 9, comprising the additional step of
removing the second sol gel coating from the preform body after hot
isostatic pressing of the body has been completed.
11. The process according to claim 9, wherein the first sol comprises a
mixture of a high-temperature sol and a high-temperature filler.
12. The process according to claim 9, wherein the second sol comprises a
mixture of a high-temperature sol and a low-temperature filler.
13. A method for encapsulating a preform prepared from a powder with glass
for the purpose of preparing a shaped body by hot isostatic pressing
comprising the steps of:
preparing a mixture comprising a sol and a filler glass having an identical
composition as the sol which gels on application to a preform
coating the outer surface of the preform with a barrier layer;
coating the preform with at least one layer of the sol mixture;
coating the preform with at least one layer of the sol mixture;
after the sol had gelled, drying the sol gel coating;
baking out the coated preform;
heating the coated preform under vacuum and at a temperature sufficient to
cause the sol gel coating to seal the outer surface of the preform; and
subjecting the preform to hot isostatic pressing at elevated temperatures
and pressures in an inert atmosphere and:
recovering the product.
14. The process according to claim 13, wherein the barrier layer comprises
boron nitride.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the production of complex components made of
metallic or ceramic materials, with powders being used as the starting
materials. This invention more particularly relates to an improved process
for encapsulating a shaped body in preparation for hot isostatic pressing
by coating the body with glass using a sol-gel technique.
2. Description of the Related Art
Powders are used as a starting material in numerous production methods in
the metallurgical and ceramic industry. Many manufactured shapes, bodies
and parts begin with a fine metallic or ceramic powder. At the start of
the manufacturing process, these powders are normally consolidated into a
shaped body, generally referred to as a "green body" or "preform."
Preforms are made by a variety of known techniques, such as die pressing,
cold isostatic pressing, slip casting, extrusion, or injection molding. In
the course of the manufacturing process, the starting powders are
typically combined with additives, such as binders, lubricants, and
sintering aids. Preforms are produced in a shape and size that is close to
the desired final shape and size of the body or part being manufactured.
The preforms, however, are often mechanically weak and porous.
One common method for consolidating the preform to the required density and
shape is known as hot isostatic pressing ("HIP"). During the HIP process,
a preform is placed in a pressure vessel, wherein it is heated at a high
temperature under high pressure, using a gas such as argon or nitrogen.
HIP units frequently operate at temperatures in excess of 2000.degree. C.,
and at pressures of over 200 MPa.
Because the HIP process is conducted at high temperature and pressure at
the same time, the process allows for the production of shapes, bodies and
parts that would be difficult, if not impossible, to produce using more
conventional means, such as sintering. Since the preform is porous,
however, and since the HIP process requires the use of high pressure gas,
steps must be taken to prevent the gas from entering the porous preform
body. The preforms, therefore, must be sealed with a gas-tight capsule
prior to pressurization during the HIP process. In addition, the capsule
must be capable of withstanding the high temperatures and pressures
associated with the HIP process.
Others have attempted to solve the problem of preform encapsulation by
utilizing capsules made of either metal or glass. One such method for
encapsulating a preform is known as metal canning. This method requires
the fabrication of a metal can in a shape close to the desired shape of
the body being manufactured. The metal can is filled with powder and
sealed under vacuum. The metal canning technique is problematic, however,
when the body being produced is complex in shape or size, because of the
difficultly and added expense associated with fabricating a can having
similar physical features.
Another known method for encapsulating a preform is by placing the preform
in a glass capsule, and then sealing the capsule under vacuum. Like the
metal canning method, however, the glass encapsulation method is also
difficult to use when the porous preform body has an intricate or uncommon
shape.
Still another method for encapsulating a porous preform is to place the
preform in a bed of glass powder, and heat the powder until the glass is
liquified. After the preform is completely immersed in the liquid glass,
the pressure from the HIP may be applied. The increase in cost and
complexity associated with this method, however, make the glass bath
method undesirable.
It is also known to encapsulate a preform by coating the preform with a
glass slurry, and, following the coating, heating the preform under vacuum
to drive off the binder and produce a gas tight coating. The glass slurry
method, however, has proven to be unreliable in that while the coating is
drying, cracks in the coating are likely to occur. Moreover, defects are
liable to occur during the removal of the binder by thermal decomposition.
Finally, the glass slurry method also adds to the complexity and cost of
the manufacturing process.
A sol-gel method may be utilized to produce a thin film of glass. A sol-gel
method generally comprises hydrolyzing and polycondensating a metal
alkoxide in a solution containing the metal alkoxide, water and an alcohol
to form a porous solid material, and heating the porous solid material to
produce a glass material. U.S. Pat. No. 5,368,887 (Hoshino), for example,
describes one type of process for producing a thin glass film by a sol-gel
method. The '887 patent, however, neither shows nor suggests the use of
the sol-gel derived glass for encapsulating a preform for the HIP process.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide an improved
process for encapsulating a preform for the purpose of hot isostatic
pressing by utilization of a sol-gel method.
Another object of the present invention is to provide a process of
encapsulating a preform that is more versatile than conventional methods,
and which may be used with a wide variety of materials, sizes and shapes.
A further object is to provide a process for encapsulating a preform that
is simple and less expensive than existing methods.
Other objects and advantages of the present invention will become readily
apparent to those skilled in this art from the following detailed
description, wherein the preferred embodiments of the invention are shown
and described simply by way of illustration of the best mode contemplated
for carrying out the invention.
DETAILED DESCRIPTION OF THE INVENTION
According to the present invention, the foregoing objectives and advantages
are attained by a process for encapsulating a preform with a sol-gel
derived glass, comprising the steps of: coating an outer surface of the
preform with one or more layers of a sol mixture, whereupon the sol
mixture gels; drying the gel; baking out the preform; and finally, heating
the preform under vacuum at a temperature sufficient to seal the sol gel
coating to the preform. Once the sol-gel coating is sealed, the preform is
capable of withstanding the relatively high pressures and temperatures
associated with consolidation during the HIP process. After the body that
has been formed by the HIP has become sufficiently cooled, the sol-gel
coating may be removed from the formed body.
The process of the present invention may be employed with any of the
conventional powders used to make a preform. Preferred examples include
titanium nitride, silicon nitride, alumina, and silica. In one example, a
preform was initially formed by die pressing the starting powder. After
the die pressing, the starting powder was then cold isostatically pressed
at approximately 22,000 psi. A number of preforms were produced in the
shape of right circular cylinders of approximately 3/8 inch diameter,
with lengths varying from 1/4 inch to 1/2 inch. Other preforms were
produced in the shape of rectangular bars, whose dimensions were
approximately 15 mm.times.2 mm.times.6 mm. These rectangular preforms were
formed by slip casting. The technique of the present invention, however,
is not limited to any particular size or shape of preform. Indeed, neither
the chemical composition of the preform, nor the manufacturing process
used to produce the preform, is pertinent to the inventive method.
The first step of the present invention comprises producing the sol. The
sol may be produced by any conventional technique, and may be either a
"high temperature" sol, or a "low temperature" sol (the significance of
these two types of sols is described more fully below). For example, one
method for making a "high temperature" sol is to mix water and alcohol at
60.degree. C., and add thereto tetraethoxysilane (TEOS). This mixture is
then acid-catalyzed, with continuous mixing at 60.degree. C. for
approximately one hour.
The next step comprises coating the entire outer surface of the preform
with the sol. Because the sol is initially in the form of a liquid, the
sol can be applied to a preform of virtually any size, dimension, or
shape. The preform may be coated by any means for applying a liquid to a
formed body. Preferred means for applying a coating of sol to the preform
include dipping, spraying, or painting.
Once applied, the sol gels to the outer surface of the preform, preferably
at room temperature (approximately 25.degree. C.) and atmospheric
pressure. After the sol has gelled, the sol gel dries at a temperature
preferably ranging from approximately 25.degree. C. to 90.degree. C. The
dried sol gel forms a hard, porous layer of glass that encapsulates the
preform in a three-dimensional, interlocked network, often referred to as
a "matrix." The matrix adheres to the surface of the preform. Additional
layers of sol-gel derived glass may be applied to the preform by repeating
the steps set forth above.
After drying, the coated preform is next placed in a hot isostatic press,
wherein it is heated under vacuum conditions to a temperature sufficient
to allow for the "baking out" of any binders or lubricants in the preform
through the pores of the glass. A preferred bake-out temperature ranges
from approximately 500.degree. C. to 800.degree. C.
After the bake-out step, the temperature in the hot isostatic press is
raised to a level sufficient to allow the pores of the glass to close,
thereby sealing the preform in a gas-tight capsule. The capsule acts as a
pressure-transfer membrane, and must be able to withstand the high
temperatures and pressures of the hot isostatic press. A preferred sealing
temperature ranges from approximately 800.degree. C. to 1500.degree. C.
After the preform has been sealed, the pressure in the hot isostatic press
is raised to that required for consolidating the preform into the desired
body shape (the "pressure step"). A preferred pressure during the pressure
step ranged from approximately 8000 psi to as high as 60,000 psi. During
the pressure step, the temperature in the hot isostatic press may be
approximately the same as the temperature during the sealing step. In
general, however, the preferred pressure step temperature ranged from
approximately 1400.degree. C. to 1900.degree. C. The hot isostatic press
is maintained under constant conditions of temperature and pressure for
approximately one hour, after which the formed body is removed from the
press and allowed to cool under ambient conditions of temperature and
pressure. Once cooled, the glass coating may then be removed from the
formed body.
In another preferred embodiment of the present invention, a step of adding
a filler to the sol is performed after the step of mixing the sol, but
prior to the step of coating the preform. The filler may be either a
"high-temperature filler" or a "low-temperature filler." The application
of the different temperature fillers is explained below. Preferred
examples of high-temperature fillers are: "VYCOR" scrap ground, then
sieved to -325 mesh; fused silica scrap ground, then sieved to -325 mesh;
and -325 mesh silica powder. Preferred examples of low-temperature fillers
are formed beginning with a starting powder, such as silicon dioxide,
aluminum oxide, boron oxide, or sodium carbonate. It should be understood,
however, that other starting materials may be used to make an acceptable
low-temperature filler.
The low-temperature filler starting powder is dry mixed, heated to
1200.degree. C. for approximately four hours, and quenched with water.
Finally, the resulting low-temperature filler material is ground, then
sieved to -325 mesh. It is noteworthy that the invention is not limited to
these particular high-temperature and low-temperature fillers; many
compositions will seal the preform at the desired temperature.
When used with a filler, the matrix functions as a "binder." The matrix
binder is preferable in that the matrix need not be removed by baking out
prior to the sealing step, since the matrix becomes part of the capsule.
Each layer may be made of either sol-gel derived glass or a combination of
sol-gel derived glass and filler. The composition of both the sol-gel
derived glass and the filler can be tailored to suit the HIP operating
environment (bake-out step temperature, sealing step temperature, pressure
step temperature and pressure), as well as for compatibility with the
preform material.
A slight modification to the inventive process must be employed when
utilizing a hot isostatic press that can not operate at high temperature
under vacuum, or when utilizing preforms made of materials that break down
at high temperature under vacuum. In either of these two scenarios, the
capsule must be sealed at a temperature that is much lower than the
pressure step temperature (for example, where the bake-out step is
conducted at 500.degree. C., the sealing step is conducted at 800.degree.
C., and the pressure step at 1500.degree. C.).
Therefore, in another embodiment of the present invention, there is
provided a first sol mixture and a second sol mixture. The first sol
mixture may comprise solely a high-temperature sol, or the mixture may
comprise a "high-temperature" sol and a "high-temperature filler"
(examples of which have been provided above). The preferred second sol
mixture comprises a mixture of "high-temperature" sol and a
"low-temperature filler." An alternative second sol mixture comprises
low-temperature sol, which may be (but need not be) mixed with a
low-temperature filler.
The preform is then coated with at least one layer (the "inner layer") of
the first sol. After the first sol has gelled and dried, the preform is
then coated with at least one layer (the "outer layer") of the second sol.
For a pressure step temperature of 1500.degree. C., for example, the inner
layer could be comprised of sol-gel derived silicon dioxide with a high
temperature filler, and the outer layer could be comprised of sol-gel
derived silicon dioxide with a low temperature filler.
The composition of the low temperature filler in the outer layer is such
that it is of low enough viscosity at 800.degree. C. (the sealing step
temperature) that it seals the pores in the silicon dioxide outer layer,
but does not seal the pores at 500.degree. C. (the bake-out temperature).
The silicon dioxide matrix of the outer layer prevents the melted fill
from running off of the preform at the sealing step temperature. As the
temperature is increased above the sealing step temperature, the amount of
silicon dioxide in the melt increases, increasing the viscosity at that
temperature, which prevents the glass from running off of the preform at
the pressure step temperature.
After the preform has been coated with the first and second gels, the
preform is placed in the HIP chamber, and the process continues with the
bake-out step, sealing step, and pressure step, as set forth above.
In yet another preferred embodiment of the present invention, a step of
coating the outer surface of the preform with a barrier layer is performed
immediately prior to the step of coating the preform with sol. The barrier
layer allows for easier removal of the glass capsule from the formed body
after cooling. This additional step can be performed regardless of whether
a filler material is added to the sol. A preferred barrier layer material
is boron nitride.
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